Deflectable catheter with a high modulus fiber puller element

ABSTRACT

A deflectable catheter includes a catheter body, an intermediate section, a tip electrode and a control handle for deflecting the catheter by manipulation of a puller element that is made of a high modulus fiber material and extends through the catheter body and the intermediate section. The proximal end of the puller element is housed in the control handle and the distal end of the puller element is in a wrapped configuration and engages with a distal tip member or the intermediate section.

FIELD OF INVENTION

The invention is directed to a deflectable catheter, in particular, adeflectable catheter with puller element.

BACKGROUND

Electrode catheters have been in common use in medical practice for manyyears. They are used to stimulate and map electrical activity in theheart and to ablate sites of aberrant electrical activity.

In use, the electrode catheter is inserted into a major vein or artery,e.g., femoral artery, and then guided into the chamber of the heartwhich is of concern. Within the heart, the ability to control the exactposition and orientation of the catheter tip is critical and largelydetermines how useful the catheter is.

Deflectable catheters are generally well-known. A convention catheterhas a control handle with a housing having a piston chamber at itsdistal end. A piston is mounted in the piston chamber and is affordedlengthwise movement. The proximal end of the catheter body is attachedto the piston. A puller wire is attached to the housing and extendsthrough the piston and through the catheter body. The distal end of thepuller wire is anchored in the intermediate section of the catheter. Inthis arrangement, lengthwise movement of the piston relative to thehousing results in deflection of the catheter intermediate section.

The puller wire is typically made of a single, non-stranded metal wire,such as stainless steel or Nitinol, and is coated with Teflon (R) or thelike. The coating imparts lubricity to the puller wire so that it canslide freely within a compression coil extending through the catheterbody. The puller wire typically has a diameter ranging from about 0.006to about 0.010 inch.

A typical method of anchoring the puller wire to the intermediatesection of the catheter is by fixedly attaching, e.g., crimping, ananchor to the distal end of the puller wire. The anchor is then weldedin a blind hole in a tip electrode.

Alternatively, the distal end of the puller wire is secured to thedistal section of the catheter tubing. A short piece of tubularstainless steel, e.g., hypodermic stock, is fitted over the distal endof the puller wire and crimped to fixedly secure the puller wire. Thedistal end of the tubular stainless steel is fixedly attached, e.g., bywelding, to a stainless steel crosspiece, which fits into a notch formedin the outer wall of the tubing. The portion of the notch not occupiedby the crosspiece is filled with glue, or the like, such as apolyurethane glue that is harder than the material of the flexibletubing. With conventional crimping methods, the cross-sectional area ofthe crimped portion of the puller wire can be significantly decreased,thereby excessively weakening the puller wire.

While these methods are generally effective in securing the distal endof the puller wire, both crimping and soldering tend to weaken thepuller wire, thereby subjecting the puller wire to fatigue and prematurebreakage.

Another typical method of anchoring the puller wire to the intermediatesection and the housing is to wrap the wire around an anchor pin. Thus,in general, when a puller wire is used in a wrapped configurationwhether for anchoring its distal end, or even around a pulley in acontrol handle for greater throw in the deflection of the catheter, thepuller wire tends to work-harden in a wrapped position, leading to aloss in elasticity and premature breakage.

Accordingly, there is a desire for an improved puller element that canbe manipulated by conventional control handles for deflecting cathetersand yet be suited for attachment at its distal end in a manner thatcauses less fatigue and premature wearing. It is also desirable thatsuch improved puller element retain more of its elasticity duringrepeated use and have overall better durability for use with controlhandles that provide more throw or employ pulley mechanisms tomanipulate deflection of the catheter.

SUMMARY OF THE INVENTION

In one embodiment of a deflectable catheter according to the invention,the catheter includes a catheter body, an intermediate section at thedistal end of the catheter body, a distal tip member distal theintermediate section, and a control handle at the proximal end of thecatheter body. Also included is a puller element made of a high modulusfiber material and having proximal and distal ends, which extendsthrough the catheter body. The proximal end of the puller element ishoused in the control handle and the distal end of the puller element isin a wrapped configuration that engages with the distal tip member.Manipulation of the control handle results in movement of the pullerelement to deflect the intermediate section. The distal tip member maybe, for example, a tip electrode. In one embodiment, the wrappedconfiguration of the puller element engages the intermediate sectionrather than the distal tip member.

In some embodiments, the deflectable catheter also includes a mechanicalstop on the distal tip member, such as a clamp around the pullerelement, a protrusion extending from the distal tip member, an openingin at least a portion of the distal tip member through which the pullerelement extends, and/or a mold surrounding the puller element and thedistal tip member. The mechanical stop could alternatively include anadhesive on the puller element and the intermediate section, and/or agroove on the intermediate section.

In some embodiments, the engagement of the wrapped configuration can bethe puller element tied or wrapped around at least a portion of thedistal tip member or around the mechanical stop. The engagement of thewrapped configuration may also include a knot in the distal end of thepuller element in contact with an opening or channel.

The high modulus fiber material may be nonconductive and/orsubstantially nonstretching. In one embodiment, the high modulus fibermaterial is braided. The high modulus fiber material can be, forexample, a polymeric material, High Molecular Density Polyethylene, amelt spun liquid crystal polymer fiber rope, or a spun para-aramid fiberpolymer, or a high strength ceramic fiber. In some embodiments, the highmodulus fiber material has a tensile strength in a range of about 300ksi (2,000 MPa) to 1,500 ksi (10,400 MPa), more preferably around 450ksi (3,100 MPa), and/or a tensile modulus in the range of about 5,000ksi (35,000 MPa) to about 20,000 ksi (140,000 MPa), more preferablyabout 9,750 ksi (68,000 MPa).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view showing a first embodiment of a catheterconstructed in accordance with the present invention.

FIG. 2 is a side cross-sectional view of an embodiment of a junction ofa catheter body and an intermediate section.

FIG. 3 is a longitudinal cross-sectional view of the intermediatesection of FIG.2 taken along line 3-3.

FIG. 4A is a side cross-sectional view of an embodiment of theintermediate section and a distal tip member.

FIG. 4B is a simplified side cross-sectional view of another embodimentof the distal tip member and a puller element anchored thereto accordingto the invention.

FIG. 4C is a simplified side cross-sectional view of another embodimentof the distal tip member and the puller element anchored theretoaccording to the invention.

FIG. 4D is a simplified side cross-sectional view of another embodimentof the distal tip member and the puller element anchored theretoaccording to the invention.

FIG. 4E is a simplified side cross-sectional view of another embodimentof the distal tip member and the puller element anchored theretoaccording to the invention.

FIG. 5 is a side cross-sectional view of another embodiment of thepuller element anchored to the intermediate section according to theinvention.

DETAILED DESCRIPTION

One embodiment of a deflectable electrode catheter constructed inaccordance with the present invention is shown in FIGS. 1-5. In FIG. 1,the catheter 10 comprises an elongated catheter body 12, a deflectableintermediate section 14 distal the catheter body, a tip electrode 29 ata distal end of the intermediate section and a control handle 16 at aproximal end of the catheter body.

The overall length and diameter of the catheter may vary according tothe application. A presently preferred catheter has an overall length ofabout 48 inches and an outer diameter of about 0.09 inches.

With reference to FIG. 2, the catheter body 12 comprises an elongatedtubular construction having a central lumen 18. In one preferredembodiment, the lumen has a diameter of about 0.018 inch. The catheterbody 12 is made of any suitable non-toxic material such as polyurethane.The catheter body 12 is preferably reinforced with at least one layer ofa braided mesh 15 of stainless steel or the like to increase itstorsional stiffness. Extending through the single lumen 18 of thecatheter body 12 are a lead wire 36 and a compression coil 33 throughwhich a puller element 31, e.g., a puller tensile member, extends. Whilethe catheter body 12 may be a multi-lumen body, a single lumen catheterbody may be preferred with certain applications over a multi-lumen bodybecause it has been found that the single lumen body permits better tipcontrol when rotating the catheter. The single lumen permits the leadwire and the puller element surrounded by the compression coil and othercomponents such as an electromagnetic sensor cable and/or an infusiontube to float freely within the catheter body. If such wires and tubewere restricted within multiple lumens, they tend to build up energywhen the handle is rotated, resulting in the catheter body having atendency to rotate back if, for example, the handle is released, or ifbent around a curve, to flip over, either of which are undesirableperformance characteristics.

With reference to FIGS. 2-4A, the intermediate section 14 comprises ashort section of flexible multi-lumen tubing 21. In the illustratedembodiment, a first lumen 23 carries one or more lead wires 36 and anyother components (e.g., thermocouple wires for monitoring tissuetemperature) extending along the catheter. A second lumen 22 carries thepuller tensile member 31. A third lumen 34 is available to carry anelectromagnetic sensor cable where an electromagnetic location sensor iscarried in or near the distal tip member. As understood by one ofordinary skill in the art, these lumens are interchangeable so long asthe puller tensile member extends through a lumen that is offset fromthe central axis of the tubing to facilitate deflection of theintermediate section when the tensile member is drawn distally, orotherwise manipulated by the control handle. Moreover, it is understoodthat any additional lumens may be used for fluid passage and/or carry anirrigation tube for supplying fluid to the tip electrode 29. Thus, thenumber of lumens or the size of each lumen is not critical, so long asit is sufficient to house the lead wires, puller tensile member,electromagnetic sensor cable, thermal sensors and/or irrigation tube(s)depending on the embodiment.

The tubing 21 is made of a suitable material and is preferably moreflexible than the catheter body 12. A presently preferred material forthe intermediate section 14 is polyurethane. The intermediate section 14is preferably reinforced with a metallic braided mesh 24 similar to thaton the catheter body 12 to impart the same high torque characteristicswithout appreciably increasing its bending stiffness. The tip electrode29 is at the distal end of the intermediate section.

The diameter of the intermediate section 14 is preferably the same as orslightly smaller than that of the catheter body 12. In a preferredembodiment, the diameter of the intermediate section is about 0.08-0.09inches and the length is about 3 inches.

A preferred means for attaching the intermediate section 14 to thecatheter body 12 is illustrated in FIG. 2. The proximal end of theintermediate section 14 comprises an outer circumferential notch 26 andthe distal end of the catheter body 12 comprises an innercircumferential notch 27. The notches 26 and 27 are sized such that theproximal end of the intermediate section 14 fits snugly into the distalend of the catheter body 12. The intermediate section 14 is then fixedlyattached to the catheter body by polyurethane glue or the like, creatinga seam on the outer surface of the catheter at the junction between theintermediate section 14 and the catheter body 12.

The tip electrode 29 is connected to the lead wire 36 which extendthrough the central lumen 18 of the catheter body 12 and the first lumen23 of the intermediate section 14. As better shown in FIGS. 2 and 3, theportion of the lead wire 36 and portions of any other lead wires thatmay be connected to ring electrodes carried on the intermediate section14, extending through the central lumen 18 of the catheter body 12,control handle 16 and the intermediate section 14 are enclosed within aprotective, nonconducting sheath 39, which can be made of any suitablematerial, preferably polyimide.

The proximal ends of lead wires 36 extend through the control handle andare connected to an appropriate jack or other connector which can beplugged into or otherwise connected to a suitable monitor or otherelectrophysiological equipment.

The puller tensile member 31 extends from the control handle 16 throughthe central lumen 18 in the catheter body 12 and into the second lumen22 of the intermediate section 14. The puller tensile member has adiameter preferably from about 0.006 to about 0.010 inch.

With reference to FIG. 2, within the catheter body 12, there is provideda compression coil 33 extending through the lumen 18 in surroundingrelation to the puller tensile member 31. The compression coil 33 ismade of a suitable metal, e.g., stainless steel, which is tightly woundon itself to provide flexibility, i.e., bending, but to resistcompression. The inner diameter of the compression coil 33 is selectedto be slightly larger than the diameter of the puller tensile member 31.For example, when the puller tensile member 31 has a diameter of about0.007 inch, a compression coil 33 having an inner diameter of about0.008 inch is presently preferred. The outer diameter of the compressioncoil 33 is likewise slightly smaller than the diameter of the lumen 18through which it extends. For example, if the central lumen 18 has adiameter of about 0.018 inch, the compression coil 33 preferably has anouter diameter of about 0.017 inch.

The distal end of the compression coil 33 could also be located in theproximal portion of the intermediate section 14 rather at the distal endof the catheter body 12. Such an embodiment provides added support tothe juncture of the catheter body 12 and intermediate section 14. Inaccordance with one embodiment of the present invention, the pullertensile member 31 has a wrapped configuration at its distal end that isanchored to the distal tip electrode 29 mounted on the distal end of theintermediate section. As shown in the embodiment of FIG. 4A, the tipelectrode 29 has at its proximal end a stem 290 to which the wrappedconfiguration of the puller tensile member is anchored. To that end, theproximal end of the tip electrode is notched circumferentially to formthe stem, which fits inside a cored distal end of the tubing of theintermediate section and is bonded thereto by polyurethane glue or thelike. In one embodiment (not shown), the tip electrode 29 has a totallength of about 7 mm, with the stem having a length of about 3.5 mm(i.e., about half the total length of the tip electrode. The distal endof the tip electrode 29 is generally solid with a blind hole 294 toreceive a distal end of a lead wire 36 for the tip electrode29.Anchoring the puller tensile member 31 to the tip electrode 29 servesto secure the tip electrode and enable deflection of the intermediatesection 14 through its entire length. To secure the distal end of thetensile member 31 to the stem of the tip electrode 29, the stem 290 maybe configured with a variety of structural elements to engage thetensile member. As shown in FIG. 4A, one embodiment includes amechanical stop in the form of a circumferential groove 292. The distalend of the puller element 31 that extends from the second lumen 22 ofthe tubing of the intermediate section is wrapped around the stem 290and sits within the circumferential groove 292. Although the wrappedconfiguration of the tensile member 31 is shown in this embodiment tocurve entirely around the stem, any suitable wrapped configuration, suchas, for example, forming an opened or closed fold, wind, loop, or curve,is also within the scope of the invention. The puller tensile member 31,in this embodiment, is fastened to the stem 290 in a slip-knot at itsdistal end to engage the tip electrode. The wrapped portion of thepuller tensile member 31 and the slip knot may be further bonded to thestem by polyurethane glue or the like when the tip electrode is insertedin the cored distal end of the tubing and bonded. Thus, as the pullertensile member 31 is drawn in the proximal direction, the intermediatesection which carries the tip electrode at its distal end is deflectedtoward the side of the tubing of the second lumen 22 carrying the pullertensile member.

In an alternative embodiment, as shown in FIG. 4B, the puller tensilemember 31 is in a wrapped configuration within the groove 292 of thestem 290, but is further fixed to the stem 290 by a clamp or a ring 296that squeezes the puller tensile member 31 into the groove 292 andfixedly retains the puller tensile member in the groove. The clamp 296may be made of any suitable material, such as metal, plastic, ceramic,etc. An adhesive, or mold may also be used in place of the clamp 296 tohold the puller tensile member 31 in the groove 292.

Referring to FIG. 4C, this embodiment includes a stem having a channel292′ drilled therethrough, through which the puller tensile member 31extends. The wrapped configuration of the puller tensile member 31 isformed by tying the distal end in any fashion, e.g., in a slip-knotaround the stem, or a knot with itself that is sized larger than thechannel to prevent the knot from entering the channel when the pullertensile member is drawn proximally.

In the embodiment shown in FIG. 4D, the stem 290″ of the tip electrode29″ includes a mechanical stop in the form of a projecting pin 292″,around which the puller tensile member 31 is wrapped and fixed throughsuitable means. In the embodiment shown in FIG. 4E shows the pullertensile member 31 wrapped around the stem 290′″ of the tip electrode29′″ and obstructed from slipping off the stem 290′″ by a mechanicalstop on the stem 290′″ in the form of a protrusion 292″′. The protrusion292′″ is shown projecting directly out of the stem 290′″ at a 90 degreeangle, but any angle or configuration capable of preventing slippage ofthe pulling element 31 off of the stem is also within the scope of theinvention.

As shown in FIG. 5, the mechanical stop in the form of a loop 292″″ canalso be located on the interior side wall of the intermediate section14′. The puller tensile member 31 that extends through the lumen 23 iswrapped through the loop 292′″ and to the wrapped configuration of thepuller tensile member 31 engages the loop such that movement of thepuller tensile member 31 in the proximal direction bends the flexibletubing 21, as described above.

In accordance with a feature of the invention, the puller tensile member31 is made of a high modulus fiber material, preferably having a tensilemodulus substantially in the range of 5,000 ksi (35,000 MPa) to about20,000 ksi (140,000 MPa), more preferably about 9,750 ksi (68,000 MPa),such as High Molecular Density Polyethylene (e.g., Spectra™ orDyneema™), a spun para-aramid fiber polymer (e.g., Kevlar™) or a meltspun liquid crystal polymer fiber rope (e.g., Vectran™), or a highstrength ceramic fiber (e.g., Nextel™). These materials tend to beflexible, providing suitable durability when used in wrapped engagementwith the tip electrode or around pulleys in the control handle forincreased throw. Further, they are substantially non-stretching, whichincreases the responsiveness to the manipulation of the control handle,and nonmagnetic so that they generally appear transparent to an MRI. Thelow density of the material causes it to be generally transparent to anx-ray machine. The materials can also be nonconductive to avoidshorting. Vectran™, for example, has high strength, high abrasionresistance, is an electrical insulator, nonmagnetic, is polymeric, andhas low elongation under a load.

Preferably, the puller tensile member 31 has a tensile strength rangingbetween about 300 ksi (2,000 MPa) to 1,500 ksi (10,400 MPa), morepreferably around 450 ksi (3,100 MPa). This would allow the pullertensile member 31 to have a higher strength than conventional steelpuller wires as well as a reduced cross-section. In one embodiment, thehigh modulus fiber material is braided.

Referring to FIGS. 1-5, any suitable control handle 16 which can controllongitudinal movement of the puller tensile member 31 relative to thecatheter body may be used. A preferred control handle 16, as shown inFIG. 1, is described in U.S. Pat. No. 4,960,134 and Re U.S. Pat. No.34,502, the entire disclosures of which are incorporated herein byreference. The handle has a piston 19 that controls the longitudinalmovement of the puller tensile member 31. The puller tensile member 31may be in a wrapped configuration that engages with the piston at itsproximal end in a manner similar to its fixation to the intermediatesection, discussed above. In the illustrated embodiment, proximalmovement of the puller tensile member with respect to the catheter bodydeflects the intermediate section and distal movement of the pullertensile member with respect to the catheter body returns theintermediate section to its nondeflected resting shape. Due to theadvantages provided by the puller tensile member of the presentinvention as mentioned above, the present invention is particularly wellsuited for control handles that use pulleys and the like to providegreater throw in deflection of the catheter, such as described in U.S.application Ser. No. 10/871,691, filed Jun. 15, 2004, the entiredisclosure of which is incorporated by reference herein.

It is understood that any mechanism for attaching the proximal end ofthe puller tensile member to the control handle may be used.

In the embodiment described above, the compression coil is preferablycovered by a non-conductive sheath to prevent electrical contact withthe lead wires. Moreover, a tunnel is formed in each of the glue jointssecuring the proximal and distal ends of the compression coil to thecatheter body. The tunnels provide means for passage of the electrodelead wires through the glue joints. Such a tunnel may be formed, forexample, by short pieces of polyimide tubing or the like.

It is also understood by one of ordinary skill in the art that thepresent invention is not limited to a catheter with a single pullerelement, but may include two or more puller element for bi-directionaldeflection or more complex deflection configurations, such as S-typedeflection or catheters with a preformed or preshaped distal end havingcombinations of straight and curved portions. Moreover, one of ordinaryskill in the art understands that the catheter of the present inventionis adaptable to carry an electromagnetic location sensor at or near thedistal end of the intermediate section, whereupon its sensor cable iscarried in the third lumen 34 of the intermediate section and thecentral 18 of the catheter body. Similarly the catheter of the presentinvention is adaptable to provide irrigation and infusion at the distalend of the intermediate section through the distal tip member. To thatend, infusion/irrigation fluids may be transported via a tube segmentextending in the central lumen 18 of the catheter body 12 and the thirdlumen 34 of the intermediate section 14, or a fourth lumen in theintermediate section 14, as desired. Suitable adaptations andembodiments are disclosed in U.S. Pat. No. 6,602,242, the entiredisclosure of which is hereby incorporated.

The preceding description has been presented with reference to presentlypreferred embodiments of the invention. Workers skilled in the art andtechnology to which this invention pertains will appreciate thatalterations and changes in the described structure may be practicedwithout meaningfully departing from the principal, spirit and scope ofthis invention.

Accordingly, the foregoing description should not be read as pertainingonly to the precise structures described and illustrated in theaccompanying drawings, but rather should be read consistent with and assupport to the following claims which are to have their fullest and fairscope.

1. A deflectable catheter comprising: a catheter body; an intermediatesection at a distal end of the catheter body; a distal tip member distalthe intermediate section; a control handle at a proximal end of thecatheter body; and a puller element comprising a high modulus fibermaterial and having proximal and distal ends, the puller elementextending through the catheter body and the intermediate section,wherein the proximal end of the puller element is housed in the controlhandle and the distal end of the puller element has a wrappedconfiguration in engagement with the distal tip member, and manipulationof the control handle results in movement of the puller element todeflect the intermediate section.
 2. The deflectable catheter accordingto claim 1, wherein the distal tip member is a tip electrode.
 3. Thedeflectable catheter according to claim 1, further comprising means foranchoring the wrapped configuration to the distal tip member.
 4. Thedeflectable catheter according to claim 1, further comprising a stop onthe distal tip member.
 5. The deflectable catheter according to claim 4,wherein the stop comprises a clamp on the puller element.
 6. Thedeflectable catheter according to claim 4, wherein the stop comprises aprotrusion extending from the distal tip member.
 7. The deflectablecatheter according to claim 4, wherein the stop comprises a channelformed in the distal tip member through which the puller elementextends.
 8. The deflectable catheter according to claim 4, wherein thestop comprises a mold on the puller element and the distal tip member.9. The deflectable catheter according to claim 4, wherein the stopcomprises an adhesive on the puller element and the distal tip member.10. The deflectable catheter according to claim 4, wherein the stopcomprises a groove on the distal tip member.
 11. The deflectablecatheter according to claim 1, wherein the engagement of the wrappedconfiguration comprises the distal end of the puller element tied to thedistal tip member.
 12. The deflectable catheter according to claim 1,wherein the engagement of the wrapped configuration comprises the distalend of the puller element wrapped around at least a portion of thedistal tip member.
 13. The deflectable catheter according to claim 4,wherein the engagement of the wrapped configuration comprises the distalend of the puller element wrapped around the stop.
 14. The deflectablecatheter according to claim 7, wherein the engagement of the wrappedconfiguration comprises a knot in the distal end of the puller elementextending through the channel.
 15. The deflectable catheter according toclaim 1, wherein the high modulus fiber material is nonconductive. 16.The deflectable catheter according to claim 1, wherein the high modulusfiber material is generally nonstretching.
 17. The deflectable catheteraccording to claim 1, wherein the high modulus fiber material isbraided.
 18. The deflectable catheter according to claim 1, wherein thehigh modulus fiber material comprises a polymeric material.
 19. Thedeflectable catheter according to claim 1, wherein the high modulusfiber material comprises a ceramic material.
 20. The deflectablecatheter according to claim 1, wherein the high modulus fiber materialcomprises High Molecular Density Polyethylene.
 21. The deflectablecatheter according to claim 1, wherein the high modulus fiber materialcomprises a melt spun liquid crystal polymer fiber rope.
 22. Thedeflectable catheter according to claim 1, wherein the high modulusfiber material comprises a spun para-aramid fiber polymer.
 23. Thedeflectable catheter according to claim 1, wherein the high modulusfiber material has a tensile strength ranging between about 2,000 MPaand about 10,400 MPa.
 24. The deflectable catheter according to claim 1,wherein the high modulus fiber material has a tensile modulussubstantially in the range of about 35,000 MPa to about 140,000 MPa. 25.A deflectable catheter comprising: a catheter body; an intermediatesection at a distal end of the catheter body; a control handle at aproximal end of the catheter body; and a puller element comprising ahigh modulus fiber material and having proximal and distal ends, thepuller element extending through the catheter body and the intermediatesection, wherein the proximal end of the puller element is housed in thecontrol handle and the distal end of the puller element has a wrappedconfiguration in engagement with the intermediate section, andmanipulation of the control handle results in movement of the pullerelement to deflect the intermediate section.
 26. A deflectable cathetercomprising: a catheter body; an intermediate section distal the catheterbody; a distal tip member distal the intermediate section; and a pullerelement comprising a high modulus fiber material and having proximal anddistal ends, the puller element extending through the catheter body andthe intermediate section, wherein the puller element has a wrappedconfiguration in engagement with the distal tip member.